Vascular calcifications have been associated with the development of regional vascular disease. The severity of aortoiliac calcification as seen on chest x-ray studies has been correlated with atherosclerotic risk factors, coronary artery disease, peripheral arterial disease and cardiovascular mortality. Aortoiliac calcification can be much more common than coronary calcification. The distribution of calcification in the aortoiliac system has been shown to vary significantly depending on age, sex and comorbidity. The occurrence of iliac calcification was also associated with an increased incidence of symptoms of intermittent claudication.
The speed and coverage of Computed Tomographic Angiography (CTA) allows the imaging of the arterial system in a single scan. This presents a unique opportunity to quantify vascular calcification in systemic arteries as a measure of the total atherosclerotic burden. Determining the quantity and distribution of calcium in each vessel could allow the investigation of the relationship between the distribution of vascular calcium and the occurrence of systemic vascular diseases including stroke, aneurysmal disease, mesenteric ischemia and lower extremity vascular disease.
Recently it has been suggested that quantifying the mass of calcium is more accurate and less variable than the classical Agatston score or the volume score (See e.g. Hoffmann et al. (2003) in a paper entitled “Vascular calcification in ex vivo carotid specimens: precision and accuracy of measurements with multi-detector row CT” and published in Radiology 229:375-381; Hopper et al. (2002) in a paper entitled “Comparison of electron-beam and ungated helical CT in detecting coronary arterial calcification by using a working heart phantom and artificial coronary arteries” and published in Radiology 222:474-482; Hong et al. (2003) in a paper entitled “Coronary artery calcium: accuracy and reproducibility of measurements with multi-detector row CT-assessment of effects of different thresholds and quantification methods” and published in Radiology 227:795-801; Hong et al. (2002) in a paper entitled “Coronary artery calcium: absolute quantification in nonenhanced and contrastenhanced multi-detector row CT studies” and published in Radiology 223:474-480 and/or Rumberger et al. (2003) in a paper entitled “A rosetta stone for coronary calcium risk stratification: Agatston, volume, and mass scores in 11,490 individuals” and published in AJR Am. J. Roentgenol. 181:743-748). Different methods have been proposed for quantification of the mass of coronary calcium. However, these methods are threshold and protocol-specific, and may require calibration scans to obtain comparable measurements across different scanners and protocols and to correct for varying levels of arterial contrast enhancement (See e.g. Hong et al. (2002) in a paper entitled “Coronary artery calcium: absolute quantification in nonenhanced and contrastenhanced multi-detector row CT studies” and published in Radiology 223:474-480 and/or Rumberger et al. (2003) in a paper entitled “A rosetta stone for coronary calcium risk stratification: Agatston, volume, and mass scores in 11,490 individuals” and published in AJR Am J Roentgenol. 181:743-748). While these methods allow total mass quantification per vessel and per scan, they do not include a detailed analysis of the shape, size and distribution patterns of calcium fragments. Also, contrast-enhancement is required for reviewers to differentiate arterial lumen from thrombus and other soft tissue in CT of systemic arteries. This presents an additional problem, as enhancement reduces the apparent volume of calcium fragments, potentially reducing the accuracy of mass-quantification.
Calcium quantification in the systemic arteries, as opposed to coronary arteries, also presents a unique challenge of practicality. CT studies currently performed for quantifying calcification of the coronary arteries include less than 100 slices and take 10-15 minutes to process manually, CTAs currently consist of hundreds to thousands of images, and therefore take significantly longer.
Accordingly, it would be an advancement in the art to develop new methods of quantifying the mass and distribution of vessel calcification that obtains the true mass of calcium independent of the level of arterial contrast enhancement, without the requirement of protocol-specific or scanner-specific calibration scans, and allowing a detailed analysis of calcium distribution patterns.